Pre-molded clip structure

ABSTRACT

A method for making a premolded clip structure is disclosed. The method includes obtaining a first clip and a second clip, and forming a molding material around the first clip comprising a first surface and the second clip comprising a second surface. The first surface of the first clip structure and the second surface of the second clip structure are exposed through the molding material, and a premolded clip structure is then formed.

CROSS-REFERENCES TO RELATED APPLICATIONS

Not applicable

BACKGROUND OF THE INVENTION

Some semiconductor die packages use clips to provide connections betweenelectrical terminals in a semiconductor die and a leadframe structurethat provides external connections for such packages. Clips are used inmany semiconductor die packages comprising power transistors such aspower MOSFETs.

When packaging a semiconductor die comprising a power MOSFET, thesemiconductor die can be attached to a leadframe structure. Apick-and-place tool can be used to attach a source clip to a sourceregion and attach gate clip to a gate region of the MOSFET in thesemiconductor die. A typical pick-and-place tool has a two vacuum holedesign, where one vacuum hole is for holding the source clip and anothervacuum hole is for holding the gate clip. The package is then molded ina molding material.

Although a conventional packaging method such as this one could be usedto package a semiconductor die, improvements could be made. For example,it would be desirable to improve the above method so that processingefficiency is improved and so that processing costs are reduced. Also,when mounting two separate clips to a die, there can be inconsistencieswhen aligning the clips to the source and the gate regions in thesemiconductor die.

Embodiments of the invention address these and other problems,individually and collectively.

SUMMARY OF THE INVENTION

Embodiments of the invention are directed to premolded clip structures,semiconductor die packages comprising the premolded clip structures, andmethods for making the same.

One embodiment of the invention is directed to a method comprising:obtaining a first clip and a second clip; and forming a molding materialaround the first clip comprising a first surface and the second clipcomprising a second surface, wherein the first surface of the first clipstructure and the second surface of the second clip structure areexposed through the molding material, and wherein a premolded clipstructure is thereafter formed.

Another embodiment of the invention is directed to a premolded clipstructure comprising: a first clip comprising a first surface; a secondclip comprising a second surface; and a molding material coupled to thefirst clip and the second clip, wherein the first surface and secondsurface are exposed through the molding material.

Another embodiment of the invention is directed to a semiconductor diepackage comprising: a premolded clip structure comprising a first clipcomprising a first surface, a second clip comprising a second surface,and a molding material coupled to the first clip and the second clip,wherein the first and second surfaces are exposed through the moldingmaterial; and a semiconductor die comprising a first die surface and asecond die surface, and a first electrical terminal and a secondelectrical terminal at the first die surface, wherein the first surfaceis electrically coupled to the first electrical terminal and the secondsurface is electrically coupled to the second electrical terminal.

These and other embodiments of the invention are described in furtherdetail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a side cross-sectional view of a semiconductor die package.

FIG. 2( a) shows a perspective view of a premolded clip structure.

FIG. 2( b) shows a side schematic view of a semiconductor die comprisinga vertical MOSFET.

FIG. 3 shows a perspective view of a semiconductor die packagecomprising two dice.

FIG. 4 shows a top view of the die package in FIG. 3.

FIG. 5 shows a portion of the die package in FIG. 3, without a clipstructure.

FIGS. 6-7 show exemplary process flows.

FIGS. 8-9 show portions of a clip structure that can be partiallyetched.

FIG. 10 shows a semiconductor die package according to anotherembodiment of the invention.

In the Figures, like numerals designate like elements.

DETAILED DESCRIPTION

Embodiments of the invention are directed to pre-molded clip structures,methods for making pre-molded clip structures, semiconductor diepackages including the pre-molded clip structures, and methods formaking the semiconductor die packages.

The pre-molded clip structures according to embodiments of the inventionallow gate and source connections to be made simultaneously toelectrical terminals (e.g., a source terminal and a gate terminal) in asingle die or multiple dice, since clips that couple to those terminalsare premolded together with a molding material. This can result in moreuniform solder connections, since the relative positions of such clipscan be fixed and consistent prior to when they are attached to thesemiconductor die.

In some embodiments of the invention, a pre-molded clip structure can bemade with solderable contact areas defined via a molding process or viaa combination of molding and partial-etching (e.g., half-etching)processes to allow compatibility with stamped clip options. Put anotherway, a partial or half-etching process can define solderable connectionsites at predetermined locations. This can result in optimum RDSonperformance and can facilitate the flow of a molding material under aclip connection while improving clip locking within the die package.Clip bonding processes using the premolded clip structure canadvantageously use one pick-and-place step to provide connections for asingle die or multiple dice.

In some embodiments, the premolded clip structure may use a 0.203 mm (orlarger) sheet of metal (e.g., copper), etched or stamped according to adesired design of the solderable part, and may then be molded. In someembodiments, the overall thickness of the premolded clip structure(including a molding material and clip structures) may be around 0.3 mm,or greater. A premolded clip structure according to an embodiment of theinvention can be used in any suitable type of semiconductor die packageincluding a wireless MLP (micro-lead package) structure.

For a wireless MLP type package, it is also possible to design a clipframe so that it is a high density matrix frame (˜400 units per stripfor a 70 mm frame width). Hence, it is possible to lower the cost of aclip frame and thereby compensate for any added cost resulting fromadditional clip molding and sawing processes. Another advantage of thisconcept is its adaptability for creating multiple chip modules (MCM).

Also, using the premolded clip approach, source and gate clipconnections can be defined on a frame via a molding process instead oflaying out complicated clip designs.

Conventional clip designs are unique to one or several devices withdedicated stamped and clip singulation tooling. However, with a premoldclip structure, high density etched frames can be designed according todesired layout requirements while sharing the same premold set-up andsaw singulation equipment.

Conventional clip designs can have difficulty making simultaneousconnections on a die and more so in when multiple chips are processed.With premold clips structures, two or more chips can be connected withclips in a single step.

FIG. 1 shows a side, cross-sectional view of a semiconductor die package100 according to an embodiment of the invention. The package 100comprises a semiconductor die 110 which is attached to a leadframestructure 124, and is a wireless MLP-type package.

The leadframe structure 124 comprises a die attach portion 124(a) (whichmay be a drain lead structure) comprising a die attach surface 124(a)-1proximate to the die 110. It is electrically coupled to a drain in aMOSFET in the die 110 An exterior leadframe surface 124(a)-2 may beopposite to the die attach surface 124(a)-1. The leadframe structure 124also comprises a source lead structure 124(b) including a first endportion 124(b)-1, an intermediate portion 124(b)-2, and a second endportion 124(b)-3. Portions 124(b)-1, 124(b)-2, and 124(b)-3 are in astepped configuration.

The leadframe structures 124 may be made of any suitable conductivematerial including plated and unplated metals. Suitable materials mayinclude copper.

The semiconductor die package 100 also comprises a premolded clipstructure 130. The premolded clip structure 130 comprises a first clip118 and a first molding material 128 around at least a portion of thefirst clip 118. The first molding material 128 may comprise any suitablematerial including an epoxy molding material. The first clip 118 and anyother clips may be made of any suitable material including copper. Thefirst clip 118 and any other clips may be plated or unplated.

The first clip 118 may be a source clip and may comprise a first portion118(a) which is electrically and mechanically coupled to a source regionof the semiconductor die 110 using a conductive material 122 (e.g., aconductive adhesive) such as solder or a conductive epoxy, as well as asecond portion 118(b), and an intermediate portion 118(c). The firstportion 118(a) may comprise a die attach surface 118(a)-1 and anopposite surface 118(a)-2. The second portion 118(b) is mechanically andelectrically coupled to the source lead structure 124 using a conductiveadhesive 129 such as solder or a conductive epoxy. The second portion118(b) may comprise a lead attach surface 118(b)-1 and an oppositesurface 118(b)-2.

The intermediate portion 118(c) is between the first portion 118(a) andthe second portion 118(b) of the first clip 118. The intermediateportion 118(c) may have been formed by an etching process, and istherefore thinner than the first portion 118(a) and the second portion118(b) of the first clip 118. The first clip 118 has a number ofpartially etched regions 118(d) (sometimes referred to as “half-etched”when about half of the thickness of the clip is etched away). As shownin FIG. 1, the molding material 128 fills the regions that were etchedaway to lock the first clip 118 into the molding material 128.

A second molding material 114, which may be the same or different thanthe molding material 128 in the premolded clip structure 130, may coversome or all of the premolded clip structure 130, and the semiconductordie 110. The second molding material 114 may also cover a portion of theleadframe structure 124. Because the second molding material 114 and thefirst molding material 128 are formed in separate processes, aninterface may be formed between the first molding material 128 and thesecond molding material 114 in some embodiments.

As shown in FIG. 1, in this example, the second molding material 114does not extend beyond the lateral edges of the drain lead structure124(a) and the source lead structure 124. (In other embodiments, thepackages could include leads which extend beyond the lateral edges ofthe second molding material 114.) Also, the surface 124(a)-2 and theexterior surface corresponding to the second end portion 124(b)-3 areexposed by the second molding material 114. The exposed surfaces may bemounted to conductive lands on a circuit substrate (not shown) such as acircuit board.

FIG. 2( a) shows the underside of the premolded clip structure 130. Thecross-sectional view of the premolded clip in FIG. 1 may be along theline P-P. As shown in FIG. 2( a), the die attach surface 118(a)-1 of thefirst clip 118 and the lead attach surface 118(b)-1 of the first clipare exposed through the molding material 128.

FIG. 2( a) also shows a die attach surface 136(a)-1 and a lead attachsurface 136(b)-1 corresponding to a second clip 136 which may be a gateclip. The second clip 136 may electrically connect a gate lead in thepreviously described leadframe structure and a gate region in thepreviously described die using a conductive adhesive such as solder or aconductive epoxy. Like the first clip 118, the second clip 136 may alsocomprise a first portion including the die attach surface 136(a)-1, asecond portion including the lead attach surface 136(b)-1, and anintermediate portion (covered by the molding material 128) that isthinner than the first portion and the second portion.

In the premolded clip 130, the first clip 118 and the second clip 136are separated from each other and are electrically isolated from eachother by the molding material 128. The molding material 128 binds thefirst clip 118 and the second clip 136 together so that the first clip118 and the second clip 136 can be mounted on to a corresponding sourceregion and a corresponding gate region in a semiconductor die togetherin one step and using vacuum tool element that includes one vacuum hole.This is unlike conventional processes where separate vacuum holes for aseparated first clip and a separated second clip would be needed.Consequently, embodiments of the invention provide for more efficientprocessing and can also provide for more accurate alignment of the firstand second clips 118, 136 when they are bonded to a semiconductor die,since they already in fixed positions relative to each other duringbonding.

FIG. 2( b) shows a schematic cross-section of a die comprising avertical power MOSFET. The die 110 comprising a source region S and agate region G at one surface of the die 118, and a drain region D at theopposite surface of the die 110.

Vertical power transistors include VDMOS transistors and verticalbipolar transistors. A VDMOS transistor is a MOSFET that has two or moresemiconductor regions formed by diffusion. It has a source region, adrain region, and a gate. The device is vertical in that the sourceregion and the drain region are at opposite surfaces of thesemiconductor die. The gate may be a trenched gate structure or a planargate structure, and is formed at the same surface as the source region.Trenched gate structures are preferred, since trenched gate structuresare narrower and occupy less space than planar gate structures. Duringoperation, the current flow from the source region to the drain regionin a VDMOS device is substantially perpendicular to the die surfaces. Inembodiments of the invention, the semiconductor dice could alternativelyinclude other vertical devices such as resistors as well as bipolarjunction transistors.

FIG. 3 shows a perspective view of a semiconductor die package 200comprising two dice 210 within a single package. The semiconductor diepackage 200 comprises two first clips 218 and two second clips 236. Thetwo first clips may be source clips coupled to source regions in thesemiconductor dice 210. The two second clips 236 may be gate clipscoupled to the gate regions in the semiconductor dice 210. Thesemiconductor dice 210 may be mounted on a leadframe structure 224.

A first molding material 228 may couple the first clips 218 and thesecond clips 236 together, and they may form a premolded clip structure230. For clarity of illustration, a second molding material is not shownin FIG. 3. Although two dice and two clips per die are shown in thisexample, it is understood that embodiments of the invention may includemore than two dice and/or more than two clips per dice in otherembodiments of the invention.

FIG. 4 shows a top view of the die package 200 shown in FIG. 3.

FIG. 5 shows a perspective view of the semiconductor die package shownin FIG. 3, without the premolded clip structure 230 on top for the dice210. In FIG. 5, conductive adhesives 228, 222(g), and 222(s) are shown.They may include a conductive adhesive 222(g) on a gate region and aconductive adhesive 222(s) on a source region of the semiconductor die210.

FIG. 6 shows a flowchart illustrating a method according to anembodiment of the invention. In a die attachment process, solder paste(or a solder wire) may be used to attach a semiconductor die to aleadframe structure (step 502). Then, solder paste can be dispensed orscreen printed on the surface of the semiconductor die opposite theleadframe structure (step 504). Then, the previously described premoldedclip structure may be attached to the semiconductor die (step 506).

In a separate process, the clip premolding process (step 501) can occurusing the previously described first molding material and first andsecond clips. The first and second clips may be in an array of clips.After the premolded clip structures are formed in an array, the array ofpremolded clip structures may be separated by sawing or some otherprocess (step 503).

After a separated premolded clip structure is attached to thesemiconductor die, a reflow process and an optional flux cleaningprocess can be performed (steps 508, 510). Then, a block molding processis performed (step 512) using a molding tool. In this step, a secondmolding material is formed around at least a portion of the die, theleadframe structure, and the premolded clip structure (step 512). Then,strip marking, package sawing, and test processes are performed (steps514, 516, 518).

FIG. 7 shows another flowchart illustrating another method according toan embodiment of the invention. The steps in FIG. 7 and FIG. 6 are thesame, except that an additional step of partially etching exposed copperis shown (step 507). This additional step can be further described withreferences to FIGS. 8 and 9.

FIGS. 8 and 9 show how copper clips in premolded clip structures 330including a first molding material 328 can be selectively plated withmetallic materials 354 such as noble metals or composite layerscomprising noble metals (e.g., NiPdAu). The exposed bare copper areas352 will later be partially or half-etched to create specific solderingsites (as in FIG. 8) or specific soldering pedestals on the clip (as inFIG. 9). These bare copper areas 352 are recessed after etching. Theplated NiPdAu areas 354 will protrude from the bare copper areas 352after etching. The etched copper areas 352 can facilitate mold compoundflow under the clip structures 330 and can enhance clip locking duringthe second, block molding process with the second molding material (step512 in FIGS. 6-7). FIGS. 8 and 9 also show tie bars 350 which formpathways for volatiles that can facilitate the escape of outgascomponents from solder paste during soldering process.

FIG. 10 shows a side, cross-sectional view of a semiconductor diepackage like the one shown in FIG. 1. However, in FIG. 10, the secondmolding material 114 does not extend beyond the top surface (includingsurfaces 118(a)-2, 118(b)-2) of the first clip 118, as well as acorresponding second clip (not shown). This top exposed option can use afilm or tape assisted molding process, where a top and bottom film isplaced on areas that will not receive a molding material. The moldingprocess can be used to ensure that molding material does not bleed on toexposed pads. Compared to the package shown in FIG. 1, the package shownin FIG. 10 is thinner, and a heat sink may be placed on top of thepremolded clip structure 130 to provide for improved heat dissipation.

The premolded clip structures and semiconductor die packages describedabove can be used in larger modules and systems. Such systems mayinclude cellular phones, computers, servers, etc.

Any of the above-described embodiments and/or any features thereof maybe combined with any other embodiment(s) and/or feature(s) withoutdeparting from the scope of the invention.

The above description is illustrative and is not restrictive. Manyvariations of the invention will become apparent to those skilled in theart upon review of the disclosure. The scope of the invention should,therefore, be determined not with reference to the above description,but instead should be determined with reference to the pending claimsalong with their full scope or equivalents.

A recitation of “a”, “an” or “the” is intended to mean “one or more”unless specifically indicated to the contrary.

1. A method comprising: obtaining a first clip and a second clip; andforming a molding material around the first clip comprising a firstsurface and the second clip comprising a second surface, wherein thefirst surface of the first clip structure and the second surface of thesecond clip structure are exposed through the molding material, andwherein a premolded clip structure is thereafter formed.
 2. The methodof claim 1 further comprising: attaching the premolded clip structure toa semiconductor die.
 3. The method of claim 2 wherein the semiconductordie comprises a source region and a gate region, and wherein the firstclip is a source clip that is electrically coupled to the source regionand the second clip is a gate clip electrically coupled to the gateregion.
 4. A premolded clip structure made according to the process ofclaim
 1. 5. The method of claim 1 wherein the first clip structurecomprises a third surface opposite the first surface, and wherein thesecond clip comprises a fourth surface opposite the second surface,wherein the third surface and fourth surface are also exposed throughthe molding material.
 6. The method of claim 1 further comprising:attaching the premolded clip structure to a semiconductor die; andattaching the semiconductor die to a leadframe structure.
 7. The methodof claim 6 wherein the molding material is a first molding material, andwherein the method further comprises molding a second molding materialaround the leadframe structure, the premolded clip structure, and thesemiconductor die to form a semiconductor die package.
 8. The method ofclaim 7 wherein the semiconductor die package is an MLP-type package. 9.A semiconductor die package made by the process according to claim 7.10. A semiconductor die package made by the process according to claim8. 11-20. (canceled)
 21. A method comprising: obtaining a first clip;and forming a molding material around the first clip comprising a firstsurface, wherein the first surface of the first clip is exposed throughthe molding material, and wherein a premolded clip structure isthereafter formed.
 22. The method of claim 21 further comprising:attaching the premolded clip structure to a semiconductor die.
 23. Themethod of claim 22 wherein the semiconductor die comprises a sourceregion and a gate region, and wherein the first clip is a source clipthat is electrically coupled to the source region.
 24. A premolded clipstructure made according to the process of claim
 21. 25. The method ofclaim 21 wherein the first clip comprises a third surface opposite thefirst surface, wherein the third surface is exposed through the moldingmaterial.
 26. The method of claim 21 further comprising: attaching thepremolded clip structure to a semiconductor die; and attaching thesemiconductor die to a leadframe structure.
 27. The method of claim 26wherein the molding material is a first molding material, and whereinthe method further comprises molding a second molding material aroundthe leadframe structure, the premolded clip structure, and thesemiconductor die to form a semiconductor die package.
 28. The method ofclaim 27 wherein the semiconductor die package is an MLP-type package.29. A semiconductor die package made by the process according to claim27.
 30. A semiconductor die package made by the process according toclaim 28.